Alumina (aluminum oxide Al.sub.2 O.sub.3) is generally obtained by caustic leaching of bauxite, the so-called Bayer process, as an intermediate product in the electrolytic production of aluminum. Red mud, the residue of the Bayer process, must be disposed of in considerable quantities. Red mud removal is one of the major technical and economic problems of the Bayer process. Most of the timer the closed circuit is operated with a supersaturated alumina solution at elevated caustic concentrations and temperatures. The solid and finely divided red mud must be separated from this circuit and washed out. During this process, concentration and temperature of the liquor are reduced, producing undesired sedimentation of alumina. This alumina, as well as the liquid containing caustic and additional quantities of alumina disposed of together with the red mud, account for a considerable loss which, however, can be reduced by suitable process parameter control. The lower the quantity and concentration of the residual moisture can be kept in the red mud, the lower the losses. A limited quantity of washing water can be used for recycling and/or washing out of these valuable substances. Any excess of water used, however, requires additional evaporation and evaporation capacity.
Current processes particularly rely on counter current decantation (CCD). Settlers are used for separation, thickening and washing out of red mud. The subsequent counter current decantation (CCD) consists of a series of washing stages. Usually, 5 to 7 stages, each one with 2 or more washers of at least 30 meters diameter, are used for the usual alumina capacities in the range of 1 million tons per year. Usually, these procedures require 3 to 5 m.sup.3 of water per ton of dry mud, exceeding the operational demand by 1 to 4 m.sup.3 per ton. Subsequently, the red mud is pumped into basins or lagoons, which are sometimes several miles away from the plant. Without any further treatment, the mud will finally dry to a solids concentration of 50%.
Frequently, dram filters for red mud separation by filtration are used as an alternative or supplement to counter current decantation. Vacuum filtration is performed after 3 to 4 washing stages, permitting a residual moisture of 45% with a washing water requirement of 2 to 3 m.sup.3 per ton of dried mud. In this process too, pumps, particularly high-pressure pumps for thick sludge, are used to pump the mud to a disposal pond where a final solids concentration after drying of up to 65% can be reached.
These technologies for the removal of red mud have a number of disadvantages or inconveniences. Sodalite, a sodium aluminum silicate, leaves the circuit in a solid phase with the red mud, constituting a great loss of chemicals, i. e. of caustic and alumina, which has a major effect on the production costs. Furthermore, plant efficiency is affected by separation of red mud in relatively large equipment and the resulting elevated holding time. Temperature and caustic concentration are reduced which leads to a reduction of liquor stability, resulting in a considerable and undesired loss of dissolved alumina; this, in turn, affects plant productivity and consequently the overall costs. This loss of soluble alumina may affect the supersaturation of process streams and, in consequence, the liquor and plant productivity, thus increasing the specific fixed costs. The final solids concentration of the red mud to be disposed of is still insufficient with 0.8 m.sup.3 to 1.5 m.sup.3 of liquor per ton of dry mud being wasted. Although the suspension has been diluted by the washing water in counter current, considerable quantities of soluble caustic and alumina are disposed of as waste into the red mud ponds. The excess water used for washing requires additional steam and evaporating capacity in order to maintain temperature and concentration in the circuit, thus increasing production and capital costs. At a solids concentration of 30 to 55%, the red mud remains in a thixotropic range with a large quantity of free caustic, which poses serious handling and storage problems.
None of the known technologies has been able to solve the problem of solid phase losses resulting from the reactive silica content in bauxite.